This invention pertains to the production of mats of glass fibers. In a conventional manner continuous primary filaments of glass are fed into an intensely hot, high speed gaseous blast which attenuates the glass into fine fibers. A battery of burners is utilized to produce the generally horizontal blasts, and the gaseous streams of fibers are conducted via forming tubes to be discharged onto a moving foraminous collection surface. Generally, as shown in Canadian Pat. No. 980,969, a common forming tube is employed although, as disclosed in U.S. Pat. No. 3,076,236, a battery of fixedly mounted forming tubes can be utilized to deliver fibers into the region of the collection surface. On each side of the forming tube, adjacent the discharge end, is located a high pressure binder header having a plurality of stationary, longitudinally extending atomizing nozzles. Binder flow to each of the nozzles is individually controlled by suitable valve means. In operation the nozzles cause the binder solution to be atomized into a cloud of mist through which the fibrous stream passes. In seeking an even distribution of binder in the resultant fiber glass mat under the conventional method, individual nozzles are selectively shut off or turned on until an adequate spray pattern is achieved, that is, until binder is evenly and adequately dispersed onto the glass fibers.
Although the pot melt rate, pull roll speeds and burner pressures of a typical fiber mat producing apparatus may be uniformly set, there is never actually complete uniformity of fiber production across the machine hot-end because of the inevitable fine but appreciable distinctions between each of the fiber generating means. Another relevant machine characteristic is shown by the fact that the stream of high velocity gases and induced air has a considerable amount of energy which causes a high degree of turbulence within the forming tube as well as channeling of the flow path through the collection surface. One effect of the complicated gaseous flow pattern and of the variations in fiber generation across the machine hot-end is that deposition of glass fibers upon the moving collection surface is not uniform.
Under prior methods, in order to assure that manufacturing specifications are adhered to and to curtail scrap losses, particularly upon machine start-up and on change-over from one product to another (having different specifications), the undulating profile of a non-uniform mat was leveled out by appropriate manipulation of the various operational parameters. For example, burner pressures were varied in an attempt to increase deposits in the region of troughs and to decrease deposits upon the thicker regions of the mat. This was a complex procedure since a change in burner pressure at one burner, for example, a center burner, did not necessarily impart a corresponding mat thickness change at the mat center, or at any predetermined location, because of the changes in velocity and direction that were induced within the forming tube by a change in burner pressure. Also illustrative of the complexity of control techniques under the prior art is the phenomenon by which, in changing from one product to another of a different density, uniform pressure changes across the battery of burners would not necessarily result in a second product having a mat of uniform thickness.
Binder spray may also have an effect upon the lay of the fibers in the formation of the mat so that individual adjustment of the binder nozzle valves often resulted in difficulty in balancing the mat profile.
The aforementioned control methods have proved to be imprecise and often haphazard, and have been used with limited success only by those machine operators who have, through experience, developed the art of manipulating binder nozzles and burner pressures. Since a change in burner pressure necessarily results in a change in fiber diameter, the manipulation of burner pressures always involved the risk of moving fiber diameters out of specifications; the maintenance of a uniformity of fiber diameters being of great concern in the manufacture of fiber glass insulation and in particular in the making of filtration media. Manipulation of binder spray nozzles to achieve uniform fiber distribution impaired uniformity of binder distribution in the resultant mat.
Whenever there is a region of non-uniformity present with the body of a filter medium, whether due to variations in the density of material, fiber diameter or percentage of binder, the effective life and performance of the filter is sharply curtailed. Thus the fiber glass filter industry seeks to manufacture products having uniform characteristics within reasonably narrow tolerances.